“On board diagnostics” (OBD) in the context of a motor vehicle is a generic term to describe the self diagnostic and reporting capability of the vehicle's systems provided by a network of sensors linked to a suitable electronic management system. Early examples of OBD systems would simply illuminate a malfunction indicator light if a problem were detected, but it provided no information on the nature of the problem. More modern OBD systems use a standardised digital connection port and are capable of providing information on standardised diagnostic trouble codes and a selection of real-time data, which enable rapid problem identification and resolution of a vehicle's systems.
Current OBD requirements require that a driver must be notified in case of a malfunction or deterioration of the emission system that would cause emissions to exceed mandatory thresholds. So, for example, the OBD limits for Euro 4: 98/69/EC for passenger diesel vehicles (category M vehicles as defined by 70/156/EEC) are: carbon monoxide (CO)—3.2 g/km; hydrocarbons (HC)—0.4 g/km; nitrogen oxides (NOx)—1.2 g/km; and particulate matter (PM) 0.18 g/km. For passenger petrol (gasoline) vehicles, the Euro 4 limits are: CO—3.2 g/km; HC—0.4 g/km; NOx—0.6 g/km; and PM—no limit.
Future vehicular emissions legislation, especially in US and Europe, requires higher sensitivity in diagnostic function so as continuously to monitor the ability of an exhaust system aftertreatment catalyst to meet the emission legislation. For example, the current draft OBD limits for Euro 5: 715/2007/EC for compression ignition (diesel) passenger vehicles are: CO—1.9 g/km; non-methane hydrocarbons (NMHC)—0.25 g/km; NOx—0.54 g/km; PM—0.05 g/km; and for positive ignition (gasoline) passenger vehicles: CO—1.9 g/km; NMHC—0.25 g/km; NOx—0.54 g/km; and PM—no limit.
In US it is understood that the OBD II legislation (Title 13, California Code Regulations, Section 1968.2, Malfunction and Diagnostic System Requirements for 2004 and Subsequent Model-Year Passenger Cars, Light-Duty Trucks and Medium-Duty Vehicles and Engines) for catalyst monitoring of gasoline/spark ignited engines requires a malfunction signal where the average Federal Test Procedure (FTP) test for NMHC conversion efficiency of a monitored portion of a catalyst system falls below 50%.
Current OBD systems for catalytic exhaust gas aftertreatment systems have limited resolution of exhaust gas sensor-based signals. Depending on whether the exhaust system is for a compression ignition or positive injection vehicle, there are a number of reasons for this including electronic noise, the fact that the sensitivity of the sensor changes with temperature and perturbation around a set point. A significant reason for this problem is that the signal:noise ratio is too low. However, it can be seen that the new legislation requires that the detection of relatively small changes in catalyst activity.
As is well known in the art, the quantity of carbon monoxide (CO), unburned hydrocarbons (HC) and nitrogen oxides (NOx) emitted when gasoline fuel is combusted in a spark-ignited internal combustion engine is influenced predominantly by the air-to-fuel ratio in the combustion cylinder. An exhaust gas having a stoichiometrically balanced composition is one in which the concentrations of oxidising gases (NOx and O2) and reducing gases (HC and CO) are substantially matched. The air-to-fuel ratio that produces the stoichiometrically balanced exhaust gas composition is typically given as 14.7:1.
Theoretically, it should be possible to achieve complete conversion of O2, NOx, CO and HC in a stoichiometrically balanced exhaust gas composition to CO2, H2O and N2 and this is the duty of a so-called three-way catalyst. Ideally, therefore, the engine should be operated in such a way that the air-to-fuel ratio of the combustion mixture produces the stoichiometrically balanced exhaust gas composition.
Another way of defining the compositional balance between oxidising gases and reducing gases of the exhaust gas is the lambda (λ) value of the exhaust gas, defined according to equation (1) as:Actual engine air-to-fuel ratio/Stoichiometric engine air-to-fuel ratio,  (1),wherein a lambda value of 1 represents a stoichiometrically balanced (or stoichiometric) exhaust gas composition, wherein a lambda value of >1 represents an excess of O2 and NOx and the composition is described as “lean” and wherein a lambda value of <1 represents an excess of HC and CO and the composition is described as “rich”. It is also common in the art to refer to the air-to-fuel ratio at which the engine operates as “stoichiometric”, “lean” or “rich”, depending on the exhaust gas composition which the air-to-fuel ratio generates: hence stoichiometrically-operated engine or lean-burn gasoline engine.
It should be appreciated that the reduction of NOx to N2 using a TWC is less efficient when the exhaust gas composition is lean of stoichiometric. Equally, the TWC is less able to oxidise CO and HC when the exhaust gas composition is rich. The challenge, therefore, is to maintain the composition of the exhaust gas flowing into the TWC at as close to the stoichiometric composition as possible.
Of course, when the engine is in steady state it is relatively easy to ensure that the air-to-fuel ratio is stoichiometric. However, when the engine is used to propel a vehicle, the quantity of fuel required changes transiently depending upon the load demand placed on the engine by the driver or during so-called “fuel cut” operation (fuel is cut to the engine when the engine management system detects that the driver has lifted off the accelerator, so only air is introduced into an engine cylinder). This makes controlling the air-to-fuel ratio so that a stoichiometric exhaust gas is generated for three-way conversion particularly difficult. In practice, the air-to-fuel ratio is controlled by an engine control unit, which receives information about the exhaust gas composition from an exhaust gas oxygen (EGO) (or lambda) sensor: a so-called closed loop feedback system. A feature of such a system is that the air-to-fuel ratio oscillates (or perturbates) between slightly rich of the stoichiometric (or control set) point and slightly lean, because there is a time lag associated with adjusting air-to-fuel ratio. This perturbation is characterised by the amplitude of the air-to-fuel ratio and the response frequency (Hz).
The active components in a typical TWC comprise combinations of platinum, palladium and rhodium supported on a high surface area oxide.
When the exhaust gas composition is slightly rich of the set point, there is a need for a small amount of oxygen to consume the unreacted CO and HC, i.e. to make the reaction more stoichiometric. Conversely, when the exhaust gas goes slightly lean, the excess oxygen needs to be consumed. This was achieved by the development of the oxygen storage component which liberates or absorbs oxygen during the perturbations. The most commonly used oxygen storage component (OSC) in modern TWCs is cerium oxide (CeO2) or a mixed oxide containing cerium, e.g. a Ce/Zr mixed oxide.
A typical sensor arrangement for a modern TWC is to dispose a first lambda sensor for contacting exhaust gas on an upstream side of the TWC and a second lambda sensor for contacting exhaust gas on a downstream side of the TWC, i.e. to contact exhaust gas leaving the TWC. The first sensor is used to control the air-to-fuel ratio of the engine by closed loop control by inputting the sensor reading to an engine control unit. Principally, the second sensor is used for two purposes: to “trim” the control of the air-to-fuel ratio of the engine, which is the primarily the purpose of the first lambda sensor; and for use in on board diagnostics.
Lambda sensors are expensive and it has been suggested recently to remove one lambda sensor and run the system on a single lambda sensor disposed within the TWC (see for example WO 2005/064139, the entire contents of which is incorporated herein by reference). Not only can this make the system overall less costly, but it is believed that, by locating the single lambda probe more intimately with the TWC, it is possible to reduce the time lag associated with adjusting air-to-fuel ratio, to control the lambda value of the exhaust gas more accurately and thereby increase the conversion efficiency. It may even be possible to use smaller TWCs comprising less of the expensive precious metal active components.
In our PCT/GB2007/050087 (the entire contents of which is incorporated herein by reference) we disclose an exhaust system for a spark-ignited internal combustion engine comprising (a) a three-way catalyst composition including an oxygen storage component coated on a flow-through monolith substrate, which substrate comprising a plurality of channels, each channel having a length extending from an inlet end to an outlet end; and (b) a single lambda sensor, wherein the substrate comprises a portion of the plurality of channels wherein the TWC composition in at least a part of the length of channels extending from the inlet end has a reduced oxygen storage activity, or no oxygen storage activity, relative to the TWC composition in a remainder of the substrate, the arrangement being such that the single lambda sensor is contacted substantially only with exhaust gas that has first contacted the TWC composition having a reduced oxygen storage activity or no oxygen storage activity.
US 2006/0165567 (the entire contents of which is incorporated herein by reference) discloses a different arrangement from PCT/GB2007/050087, wherein a partial volume comprising all passages and their walls extending from a front face of a substrate—as opposed to merely a portion thereof—has a lower capacity to take up oxygen. Furthermore, in US 2006/0165567 the lower capacity to take up oxygen feature in the partial volume can be provided by no washcoat at all rather than a different formulation of the washcoat itself, as disclosed in PCT/GB2007/050087.
Exhaust systems for vehicular lean-burn internal combustion engines (diesel or gasoline) comprising a device for absorbing NOx from lean exhaust gas and releasing the stored NOx in an atmosphere containing less oxygen for reduction to dinitrogen (N2) are known from, for example, EP 0560991 (the entire contents of which is incorporated herein by reference). Such NOx-absorbents are typically associated with a catalyst for oxidising nitrogen monoxide (NO) to nitrogen dioxide (NO2), e.g. platinum (Pt), and, optionally, also a catalyst for reducing NOx to N2, such as rhodium. A catalyst comprising the NOx-absorbent and a NO oxidation catalyst and optional NOx reduction catalyst is often called a lean NOx-trap or simply a NOx-trap.
NOx-absorbents in a typical NOx-trap formulation can include compounds of alkali metals, e.g. potassium and/or caesium; compounds of alkaline earth metals, such as barium or strontium; and/or compounds of rare-earth metals, typically lanthanum and/or yttrium. One mechanism commonly given for NOx-storage during lean engine operation for this formulation is that, in a first step, the NO reacts with oxygen on active oxidation sites on the Pt to form NO2. The second step involves adsorption of the NO2 by the storage material in the form of an inorganic nitrate.
When the engine runs intermittently under enriched conditions, or the exhaust gas is at elevated temperatures, the nitrate species become thermodynamically unstable and decompose, producing NO or NO2. Under enriched conditions, these NOx are reduced by carbon monoxide, hydrogen and hydrocarbons to N2, which can take place over the reduction catalyst.
Whilst the inorganic NOx-storage component is typically present as an oxide, it is understood that in the presence of air or exhaust gas containing CO2 and H2O it may also be in the form of the carbonate or possibly the hydroxide.
Selective catalytic reduction (SCR) of NOx by nitrogenous compounds, such as ammonia or urea, has been proposed for use in treating vehicular exhaust gas. Several chemical reactions occur in an NH3 SCR system, all of which represent desirable reactions that reduce NOx to nitrogen. The dominant reaction is represented by reaction (2).4NO+4NH3+O2→4N2+6H2O  (2)
Competing, non-selective reactions with oxygen can produce secondary emissions or may unproductively consume ammonia. One such non-selective reaction is the complete oxidation of ammonia, shown in reaction (3).4NH3+5O2→4NO+6H2O  (3)Also, side reactions may lead to undesirable products such as N2O, as represented by reaction (4).4NH3+5NO+O2→4N2O+6H2O  (4)
Typical SCR catalysts include transition metal/zeolites, such as Cu/ZSM-5 and Fe/beta and base metal catalysts such as vanadia, e.g. V2O5/WO3/TiO2 (see EP 1054722, the entire contents of which is incorporated herein by reference).
Lean NOx catalysts (LNCs) are sometimes also referred to in the literature as non-selective catalytic reduction (NSCR) catalysts, hydrocarbon selective catalytic reduction (HC-SCR) catalysts, lean NOx reduction catalysts, “DeNOx catalysts” and NOx occluding catalysts.
In lean NOx catalysis, hydrocarbons (HC) react with NOx, rather than oxygen (O2), to form nitrogen (N2), carbon dioxide (CO2) and water (H2O) according to reaction (5):{HC}+NOx→N2+CO2+H2O  (5)
The competitive, non-selective reaction with oxygen is given by reaction (6):{HC}+O2→CO2+H2O  (6)
A number of catalysts are known for selectively promoting the reaction (5) including platinum (Pt) on alumina (Al2O3), copper (Cu)-substituted zeolite such as Cu/ZSM-5 and silver (Ag) supported on Al2O3 (see e.g. EP 0658368, the entire contents of which is incorporated herein by reference).
Catalytic systems designed used for treating diesel exhaust gas include catalysed soot filters (CSFs) and diesel oxidation catalysts (DOCs).
In CSFs (see for example U.S. Pat. No. 5,100,632, the entire contents of which is included herein by reference), a filter substrate is coated with a catalyst material in order to combust diesel particulate matter. Typical CSF catalyst components include platinum and cerium.
A DOC (see for example U.S. Pat. No. 6,274,107, the entire contents of which is included herein by reference) is designed to oxidise CO to CO2 and gas phase hydrocarbons (HC) and an organic fraction of diesel particulates (soluble organic fraction or SOF) to CO2 and H2O. Typical DOC components include platinum and optionally also palladium on a high surface area oxide support including alumina, silica-alumina and a zeolite.